In a boiling water nuclear reactor, moderated by light water, the fuel exists in the form of fuel rods arranged in a certain, normally symmetrical pattern, a so-called lattice, and is retained at the top by a top tie plate and at the bottom by a bottom tie plate. A fuel assembly comprises one or more bundles of fuel rods which are surrounded by a fuel channel with a substantially square cross section. In the core of the reactor, the fuel assemblies are arranged vertically and spaced from each other. During operation, the water is admitted through the bottom of the fuel assembly and then flows upwards through the fuel assembly past the fuel rods. The heat emitted by the fuel rods is taken up by the water which starts boiling, whereby part of the water is transformed into steam. The water and the steam are passed out through the upper end of the fuel assembly. The produced steam is delivered to turbines which drive generators where electrical energy is generated.
A disadvantage with a boiling reactor is the high proportion of steam by volume in the upper part of the fuel assembly.
When the proportion of steam by volume rises in the coolant, its ability to carry off heat from the fuel rods is reduced, thus increasing the risk of dryout, which in turn leads to an increase of the risk of fuel damage.
Still another problem with a high steam volume in the fuel is that steam is inferior to water as moderator, which results in the moderation being insufficient whereby the fuel is utilized inefficiently. In the lower part of the fuel assembly, the moderator consists of water whereas the moderator in the upper part of the fuel assembly consists of both steam and water. This means that the fuel in the upper part of the fuel assembly cannot be utilized efficiently. It is, therefore, desirable to keep down the steam volume in the coolant while at the same time maintaining the steam generation at a high level.
The faster the steam disappears out from the fuel assembly, the lower the steam volume. A separation of the steam flow and the water flow in the upper part of the fuel assembly thus gives the steam flow a higher velocity than the water flow, whereby the proportion of steam by volume in the fuel assembly is reduced. In this way, the margin with respect to dryout is improved and the fuel in the upper part of the fuel assembly is utilized in a better way.
U.S. Pat. No. 5,091,146 discloses a fuel assembly which attempts to achieve a separation of the steam flow and the water flow in the upper part of the fuel assembly by arranging a steam pipe above one or more part-length fuel rods, that is, fuel rods extending from the bottom tie plate but terminating below and at a distance from the top tie plate. In this way, the steam which is generated in the coolant is to be diverted. The pipe has openings both in its upper and its lower end. The disadvantages of such a pipe are several. For one thing, it may be expensive to manufacture, and, for another, it gives an increased pressure drop in the upper part of the fuel assembly. Another disadvantage is that it may be difficult to cause the continuously produced steam to enter the pipe. Admittedly, the pipe is provided with openings and other devices to cause the steam to flow into the pipe and to prevent water from entering the pipe, but it is still doubtful whether this is an effective way of causing the steam to enter the tube.